1. Field of Application
The present invention relates to a motor control apparatus for a system in which a control object is driven from a current actuation position to a target position by the motive force of an electric motor
2. Description of Prior Art
In recent years, there has been an increasing trend in the field of automobile technology towards replacing mechanical systems by systems which are electrically driven, i.e., by electric motors. This is done for reasons such as reduction of the amount of space required, ease of assembly, ease of system control, etc. This is exemplified in Japanese Patent Laid-open No. 2002-323127, whereby an automatic transmission apparatus (referred to in the following simply as automatic transmission) of a vehicle is actuated to establish respective shift positions such as the drive (D) shift position, etc., through being driven by an electric motor (hereinafter referred to simply as a motor). The motor shaft is coupled through a speed reduction mechanism to an output shaft, which drives a shift position switching mechanism that directly acts on the automatic transmission to effect changeover from one shift position (i.e., shift range) to another. The motor is provided with an encoder for detecting the rotation angle of the motor shaft. When changeover of the shift position is to be performed, the motor shaft is rotated to a target value of rotation angle (expressed as a target count value of output pulses produced by the encoder) that corresponds to a target shift position. This is described in pages 3, 4 of the above-mentioned prior art patent.
However if it is attempted to more rapidly execute a shift position changeover by increasing the motor rotation speed, overshoot of a target rotation angle of the motor shaft (corresponding to the target shift position) may occur, due to the inertia of the rotor of the motor. Hence, the motor may not become halted at the required target rotation angle.
In order to be able to satisfy both the requirements for high speed of rotation of the motor and accuracy of halting the rotation the assignees of the present invention have previously proposed, in Japanese Patent Laid-open No. 2002-177739, an apparatus whereby it is determined that a motor is operating in an acceleration range, after driving of the motor has commenced, when a deviation of a detected rotation angle of the motor (obtained as a count value of a number of pulses produced from an encoder) from a target rotation angle (i.e., target count value of encoder pulses) is higher than a predetermined threshold value, while it is determined that the motor is operating in a deceleration range when the deviation is lower than the threshold value. During operation in the acceleration range, a high value is set for the target rotation speed, so that the motor will be driven to attain a high speed of rotation, while during operation in the deceleration range, a low value is set for the target rotation speed, so that the motor can be accurately halted at a target rotation angle.
However in a system in which the load imposed on a motor may vary while the motor is being driven, it is necessary to set the target rotation speed at a sufficiently low value with regard to these variations in the load. Hence, the motor operation cannot be optimized with respect to achieving a high speed of rotation.
For example, in the case of a motor-driven shift position switching apparatus for the automatic transmission of a vehicle, when changeover is performed from the P (parking) shift position to the D (drive) shift position, it is necessary for the motor to effect the changeover in the sequence:P shift position→R (reverse) shift position→N (neutral) shift position→D shift position
A detent mechanism is provided, for retaining the automatic transmission in the selected shift position. When changeover is performed for example from the P to the D shift position, then immediately before a catch member of the detent mechanism attains a tip of a circumferentially protruding portion of a detent lever, the load on the motor increases substantially. As the catch member moves over successive ones of these protruding portions of the detent lever, large variations occur in the level of motor load, i.e., with the load first increasing (as a positive-direction load) and then becoming inverted in direction (a negative-direction load), as such a protruding portion is moved over.
Hence if the target rotation speed of the motor is set in accordance with the difference between the detected rotation angle and the target rotation angle of the motor, without considering the variations in motor load, then the drive torque provided by the motor may be insufficient, when the level of motor load reaches a high level. Thus it is necessary to set a low value for the target rotation speed of the motor, in order to ensure that there will be sufficient drive torque at all times, so that an optimally high speed of rotation cannot be utilized for the motor. Furthermore, when the motor is operating under a low level of load, the drive torque may be excessively high, so that stable control cannot be achieved.